Many factors, including solar wind, the earth's magnetic field, and gravity, contribute to attitude error of satellites in earth orbit. These factors induce disturbance torques about satellite axes. The disturbance torques are often described using terms related to windmill, overturning, and pitch axes. The windmill and overturning axes lie primarily in the azimuth (roll-yaw) plane and are perpendicular to each other. The pitch axis is perpendicular to both the windmill and overturning axes. When a satellite experiences such disturbance torques, momentum wheels become loaded and the satellite may experience rotational motions about its various axes.
Attitude error caused by disturbance torques affects the level of service that a satellite can provide. For example, the attitude of a communication satellite should be precisely maintained in order to align a communication antenna with an antenna target. When attitude error is not corrected, the satellite consumes excessive power in order to transmit communications at a higher power level to compensate for antenna misalignment. In addition, attitude error causes "cosine" power loss, especially during the solstice periods of the year. Cosine power loss results from incorrectly aligning the solar panels with respect to the sun. To the extent that solar panels are not optimally aligned with the-sun, the solar panels generate less electrical power than they might otherwise
Conventional methods of damping disturbance torques involve the utilization of tanks and momentum wheels located in the satellite body to slowly absorb the disturbance. However, the disturbance loading must eventually be unloaded or the satellite attitude will suffer. When the disturbances are unloaded, attitude errors can occur.
One-axis solar sailing or tacking is often utilized to counter disturbance torques. Conventional solar tacking techniques involve extensive activations of solar panel rotation stepper motors to pivot the solar panels about the pitch axis. However, this type of solar tacking generates undesirable windmill torques which occasionally pass through a zero torque. As the windmill torque passes through zero, attitude transients would result if solar tacking were employed to correct unwanted disturbance motions. To prevent these attitude transients, conventional techniques excessively accumulate attitude error until the attitude error can be corrected without encountering a significant duration of zero windmill torque. In general, attempting to counteract a disturbance torque in only one dimension aggravates disturbance problems in the remaining two dimensions.
In addition, a conventional use of solar tacking to address attitude disturbance control involves moving solar panels up to 15 degrees or more as quickly as possible, and holding the panels in position for roughly a six-hour period before returning the panels to their prior orientation. This allows build-up of secular momentum and significant variation of cyclical momentum. Moreover, single-axis solar tacking techniques excessively prohibit solar panels from squarely facing the Sun. This results in excessive cosine power loss, especially during the solstice periods of the year.
Disturbances due to solar, magnetic, or gravitational influences or by counter-disturbance satellite motions cause another form of attitude error called nutation, primarily about the pitch axis. Nutation leads to inefficient satellite power management. Thus, a need exists to cancel out nutation to better control satellite attitude.